The invention relates to an off-shore fluid transfer system and to an associated transfer method. The fluid may be liquefied natural gas for example and the transfer may be made between two ships on the open sea.
The first of the two ships may be a production ship known by the name LNGP (for “Liquefied Natural Gas Producer”), LNG-FPSO (for “Liquefied Natural Gas Floating Production Storage and Offloading”) or FLNG (for “Floating Liquid Natural Gas Unit”), a reliquefaction ship (FSRU for “Floating Storage and Regasification Unit”), a GBS (for “Gravity Base Structure”) or lastly a platform.
The second of the two ships may be a ship adapted to receive the gas for its transport, such as a tanker or an LNG-C (for “Liquefied Natural Gas Carrier”, for example a methane tanker).
Systems are known with rigid piping articulated to two successive arms and other flexible piping systems such as cryogenic hoses. One of the two structures, often the FLNG, possesses such movable piping extending over several meters outside the perimeter of its hull, a few meters above the level of the sea, and being adapted to be connected to ducts fixed to the second structure, within the perimeter of the hull thereof, in vertical alignment with the hull, or at a slight horizontal distance therefrom. Thanks to the flexibility in three dimensions given either by the two articulations controlling the arms, or by the flexible character of the piping, the transfer of fluid may be carried out successfully on a rough sea.
Systems are known, for example from the document EP0947464, using a coupling in which the movable piping, which is articulated, comprises a fastening flange which, disposed vertically, couples by a descending movement with a connector of the second structure which is open upwardly. A complex system of counterweights or cables raised from a support structure ensures, in case of disconnection, by leverage, the spontaneous rotation upwardly of the distal part of the articulated piping around an articulation disposed at mid-length of the piping, so as to avoid striking between that distal part and the second structure. During the connection, an additional cable provides for the positioning of the fastening flange relative to the connector of the second structure. This cable is fastened to the movable piping before a final assembly of swivel joints or rotations, which results in the opening of the piping being spontaneously oriented downwards, due to gravity. The connection is complicated by this configuration, since the approach of the fastening flange in the presence of motion due to the waves is delicate.
By contrast, patent application FR 2 941 434 describes a transfer system using, for the connection of the articulated tube that comes from the first ship to the piping of the second ship, an acquisition cable (also called LNGC cable or methane tanker cable) fastened to the free end of the tube and maneuvered by a winch disposed on the second ship. This solution enables the coupling of the ducts to be carried out by a movement of the free end of the articulated tube having a rising main component, the free end then being received by a connector on the second ship the opening of which is substantially downwardly directed. Such a solution enables shocks during the connection to be avoided simply, and to establish a coupling requiring no guidance other than that given by the acquisition cable.
Nevertheless, despite the solutions proposed until now, certain situations may still be difficult to manage during maneuvers. In particular, in a situation of emergency disconnection, it is desired to avoid plunging of the free end of the movable piping into the water. Furthermore, given the fast rates necessitated by the use of the structures, it is desired to enable the two ships to connect their ducting rapidly, and to move apart from each other as fast as possible after the disconnection, whatever the circumstances thereof.